Why do we remember some dreams but not others?

WHY do we remember some dreams but not others? It's because the brain mechanism that controls whether we remember or forget things when we are awake is involved.

So say Luigi De Gennaro at the University of Rome, Italy, and his team, who used an electroencephalogram (EEG) to monitor the brain activity of students as they slept. The team monitored 65 students: 30 who habitually wake up while in rapid eye movement (REM) sleep, and 35 who usually wake in stage 2 non-REM sleep. About two-thirds of both groups recalled dreams during the study.

Those who woke during REM sleep and successfully recalled their dreams were more likely to demonstrate a pattern of EEG oscillations called theta waves in frontal and prefrontal cortex areas - the parts of the brain where our most advanced thinking occurs. "The kind of EEG oscillations and the cortical region involved are the same as those important for recalling memories in awake subjects," says De Gennaro.

In non-REM wakers, those who remembered their dreams had patterns of alpha wave activity in the right temporal lobe - involved in recognising emotional events - that resembled activity known to be key for recall while awake (Journal of Neuroscience, DOI: 10.1523/jneurosci.0412-11-2011).

The upshot is that even when we are asleep, the same parts of our brains are on the alert for things to remember. These are often events that are emotionally charged and that the brain deems important, whether we are awake or not.

De Gennero says the results are the first evidence that the physiology by which memories are stored is the same whether we are awake or asleep. "These findings are similar to known EEG patterns in wakeful memory recollection, suggesting a continuum of cerebral processes throughout the sleep-wake cycle," says Michael Czisch, who studies sleep at the Max Planck Institute of Psychiatry in Munich, Germany.